Numerous designs exist in connection with this type of boilers: see, for example, U.S. Pat. No. 4,355,602 issued Oct. 26, 1982 to Cooke as well as U.S. patent application Ser. No. 536,836 filed June 12, 1990 U.S. Pat. No. 4,993,368. However, these constructions, which mainly consist of upper and lower manifolds between which a plurality of serpentine liquid carrying tubes are mounted, do not permit the heated liquid, entering at diametrically opposite ports on the upper manifold, to be at substantially the same temperature and to have substantially the same liquid flow.